>the pilot was confused by hypoxia upon reaching a very high height and died from lack of oxygen

I'd expect it was more a matter of passing out, losing control, not ejecting because of being passed out or the violence of the spin rather than dying from hypoxia.

The P51s came with supplemental oxygen systems but they were the re-breather mask type. 25000 feet (7600m) is marginal for that kind of rig (these days you're supposed to use a diluter-demand type above 25).

So the pilot was right at the edge of what his O2 system could support; if that system wasn't functioning perfectly I can see this happening. Anoxia creeps up on you - and it's a bit like alcohol in the sense that the first thing to go is your judgement.